It can be too powerful or break the game's mechanics
If you don't reveal an Equipment card, or if the exchange can't be completed (such as if Arcanum Things is no longer on the battlefield, you don't own Arcanum Things, or the new Equipment can't be attached to the creature Arcanum Things is attached to), nothing happens
Phyrexian Metamorph can be a copy of an artifact or creature you need
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He is as well credited with designing the Shadow Lugia for Pokmon XD: Gale of Darkness, a jumbo size promo card
Because Xyz Summoning removes monsters to turn them into materials, you can Xyz Summon onto a field where every monster slot has been filled
